An ignition coil provided with a sparkplug is mounted to an engine of a vehicle. The sparkplug generates spark to ignite mixture of fuel and air in each cylinder of the engine. When the mixture is burned in the cylinder, fuel contained in the mixture is ionized, so that an ion current flows between a pair of electrodes provided to the sparkplug. An ion current detector is provided to such an ignition coil to detect an ion current for monitoring misfire in the cylinder.
The ignition coil generates therein an inductive magnetic field by terminating electricity supplied to a primary coil of the ignition coil. The inductive magnetic field generates induced electromotive force in a secondary coil of the ignition oil, so that the pair of electrodes of the sparkplug generates spark. Immediately after generating the spark by forming the inductive magnetic field, residual magnetism remains in the ignition oil. The ion current detector may falsely detect noise, which is caused by the residual magnetism, as the ion current. Accordingly, the ion current detector detects the ion current by waiting a predetermined period after generating the spark.
According to U.S. Pat. No. 5,866,808 (JP-A-H9-195913), the ion current detector has a structure capable of stably detecting the ion current by reducing residual magnetism. Furthermore, according to the ignition coil in JP-U-3028977, the cross section of the outer core is set to be in a range between 75% and 100% of the cross section of the center core, thereby enhancing ignition energy of the ignition coil.
However, the above conventional structure of each ignition coil is not sufficient to stabilize detection of the ion current. Specifically, in the above conventional structure of each ignition coil, a relationship between axial end surfaces of the center core and the outer core on the axially opposite side of the spark plug is not considered. Accordingly, the above conventional structure is not sufficient to enhance accuracy in the detection of the ion current.